Enzyme deficiencies in cellular compartments such as the golgi, the endoplasmic reticulum, and the lysosome cause a wide variety of human diseases. For example, lysyl hydroxylase, an enzyme normally in the lumen of the endoplasmic reticulum, is required for proper processing of collagen; absence of the enzyme causes Ehlers-Danlos syndrome type VI, a serious connective tissue disorder. GnT II, normally found in the golgi, is required for normal glycosylation of proteins; absence of GnT II causes leads to defects in brain development. More than forty lysosomal storage diseases (LSDs) are caused, directly or indirectly, by the absence of one or more proteins in the lysosome.
Mammalian lysosomal enzymes are synthesized in the cytosol and traverse the ER where they are glycosylated with N-linked, high mannose type carbohydrate. In the golgi, the high mannose carbohydrate is modified on lysosomal proteins by the addition of mannose-6-phosphate (M6P) which targets these proteins to the lysosome. The M6P-modified proteins are delivered to the lysosome via interaction with either of two M6P receptors. The most favorable form of modification is when two M6Ps are added to a high mannose carbohydrate.
Enzyme replacement therapy for lysosomal storage diseases (LSDs) is being actively pursued. Therapy, except in Gaucher's disease, generally requires that LSD proteins be taken up and delivered to the lysosomes of a variety of cell types in an M6P-dependent fashion. One possible approach involves purifying an LSD protein and modifying it to incorporate a carbohydrate moiety with M6P. This modified material may be taken up by the cells more efficiently than unmodified LSD proteins due to interaction with M6P receptors on the cell surface. However, because of the time and expense required to prepare, purify and modify proteins for use in subcellular targeting, a need for new, simpler, more efficient, and more cost-effective methods for targeting therapeutic agents to a cellular compartment remains.